[return: MarshalAs(UnmanagedType.LPWStr)]
[DllExport("Test", CallingConvention = CallingConvention.Cdecl)]
public static string Test(
    [MarshalAs(UnmanagedType.LPWStr)] string firstName,
    [MarshalAs(UnmanagedType.LPWStr)] string lastName)
{
    return string.Format("Hello {0} {1}", firstName, lastName);
}